Proportional force amplifiers of the type contemplated herein are used to provide equal amounts of movement between the input device and the output device at a substantially increased force output. Generally a 100 to one or greater force relationship is contemplated. The amount of input force can be affected by any forces which resist the movement of the spool valve, such as friction due to a lack of concentricity with the bore of the piston or variation in pressures which are on the ends of the spool valve. Seal failures have also been encountered where the seals are subjected to high working pressures.